Method for Producing a Grain-Oriented Electrical Steel Flat Product Intended for Electrotechnical Applications

ABSTRACT

The invention relates to a method for producing a grain-oriented steel flat product for electrotechnical applications, wherein, in a production step, “decarburising and nitriding annealing” is carried out in two stages. The first stage of the annealing process extends over a first time interval, which comprises heating the cold strip starting from a start temperature to a first target annealing temperature and holding it at this target annealing temperature, and the second stage of the annealing process extends over a second time interval, in which the cold strip is heated to a second target annealing temperature and subsequently held at this target annealing temperature.

The invention relates to a method for producing grain-oriented electrical steel flat products intended for electrotechnical applications. Such electrical steel flat products are also, in practice, referred to as grain-oriented “electrical sheets” or grain-oriented “electrical strips”.

Grain-oriented electrical steel flat products have special magnetic properties and are produced by means of an elaborate production process. The base material for electrical steel flat products is a silicon steel sheet. The metallurgical properties of the material, the deformation degrees of the rolling processes and the parameters of the heat treatment steps are coordinated such that targeted recrystallisation processes take place. These recrystallisation processes result in a “Goss texture” which is typical for the material, in which the direction of easiest magnetisability is in the rolling direction of the finished strips.

Electrical steel flat products, in which the grains do not have a distinct alignment, are to be differentiated from grain-oriented electrical sheet or strip of the kind in question here. In such non-grain-oriented electrical strip or sheet, the magnetic flux is not fixed in any specific direction, so that identical magnetic properties form in all directions (isotropic magnetisation).

Grain-oriented electrical strip or sheet of the kind in question here, in contrast, has a strongly anisotropic magnetic behaviour. This can be attributed to a uniform orientation of the grains (crystallites) of the microstructure. This crystallographic texture is achieved by means of an effective grain growth selection effected by corresponding measures in the production process. The aim is to obtain an electrical steel flat product after final annealing, which takes place at the end of the production process, in which the grains have a low misorientation and hence have an almost ideal texture.

Grain-oriented electrical strip is particularly suitable for applications, in which particularly high requirements are imposed on the magnetic properties, as is the case, example, when building transformers.

A relatively large number of methods are known for producing high-grade grain-oriented electrical sheet.

With the so-called “low-heating method” described in EP 0 910 676 B1, high permeable, grain-oriented electrical sheets can be produced having an optimised distribution of properties. This method is characterised by a slab heating temperature below 1250° C. Due to this comparatively low temperature, aluminium nitrides, which are brought fully into solution during the high-temperature annealing step carried out at the end of the production process, are only partly dissolved and precipitated again. Consequently, electrical strip produced according to the low-heating process has a weaker inherent inhibition than material produced by means of the conventional process path via high-temperature slab heating.

The purpose of particle inhibition is to suppress the grain growth in the primary microstructure of the cold strip during and after decarburisation annealing. Controlled abnormal grain growth in the temperature range from 950-1100° C. is only to take place during final coarse grain annealing, in which the cold strips are annealed at temperatures of up to 1200° C., in order to make a high texture sharpness with Goss orientation (001) (110) possible.

After decarburisation annealing, an ideal equilibrium state between driving and restoring forces has to be set, so that optimum abnormal grain growth with high texture sharpness begins. The driving force for the grain growth during coarse grain annealing is the grain boundary energy stored in the microstructure. This is essentially determined by the grain size after primary recrystallisation.

Due to the weaker inherent inhibition with the low-heating method, the average primary grain size is greater after a decarburisation annealing treatment than with the conventional method and is subject to greater fluctuations through the cold process. Hence, the driving force for the abnormal grain growth is generally lower. On the other hand, a restoring force opposing the abnormal grain growth is determined by the non-magnetic precipitations (inhibitors) precipitated in the cold strip. Therefore, it is essential to have many finely distributed particles present. In the case of the low-heating method, the relevant particles are not, however, produced in the hot strip, but before, after or during decarburisation annealing or during the heating phase of final annealing in the course of a variety of nitriding processes.

In the course of the processes described in EP 0 950 119 B1 and EP 0 950 120 B1, via the hot-rolling process an inhibition strength Iz is set by nitrides and sulphides in such a way that the primary grain growth is inhibited during the cold process even at higher temperatures. The slabs are heated to temperatures from 1100° C. to 1320° C. before hot rolling. A nitriding treatment carried out simultaneously with the decarburisation annealing treatment at temperatures between 850 and 1050° C. in an atmosphere containing ammonia enables the direct formation of aluminium nitrides. The subsequent coarse grain annealing does not have to be modified compared to the conventional production path for the manufacture of grain-oriented electrical strip.

In contrast, in the case of the method described in EP 0 219 611 B1, the nitriding is carried out after primary recrystallisation but before the abnormal grain growth begins. Here, the nitriding can be effected by means of an atmosphere having a nitriding capability or by means of a nitrogen-donating adhesion protection additive.

Specifically in the case of the method with an ammonia-containing atmosphere, in which the nitriding temperature is below 850° C., silicon-manganese nitrides are present close to the surface after nitriding (Materials Science Forum 204-206 (1996), 143-154). Due to their lower thermodynamic stability they dissolve during the heating phase of coarse grain annealing. Then, the nitrogen diffuses into the steel matrix and recombines with the free aluminium present there to form aluminium nitride (Materials Science Forum 204-206 (1996), 593-598). The aluminium nitrides formed in this was are thereupon the effective inhibitors for the secondary grain growth. Although the inhibition is weaker compared to the conventional process used for producing grain-oriented electrical sheet, it enables a full secondary recrystallisation at higher temperatures with a greater secondary grain size in the finished strip (TMS Proceedings 3 (2008), 49-54).

However, a disadvantage of this procedure is that modified time-temperature cycle of the coarse grain annealing process is required. The dissolving of the silicon-manganese nitrides and the new formation of AlN through nitrogen diffusion take place at temperatures between 700 to 800° C. In order to fully facilitate this critical process step, when carrying out the previously explained method an isothermal holding stage of as least four hours is required during the heating phase of coarse grain annealing. This not only causes the total duration of the process to be considerably lengthened but also results in increased production costs.

In addition to the previously explained prior art, a method for producing high-grade grain-oriented electrical strip based on thin slab continuous casting is known from EP 1 752 549 A1, in which the production steps are coordinated such that an electrical sheet having optimised magnetic properties is obtained using conventional aggregates. In the course of this, the aim is to prevent the formation of nitridic precipitations before and during hot rolling as far as possible, so that the possibility of producing such precipitations in a controlled manner during cooling of the hot strip can be exploited to a large extent. Specifically, firstly a steel is melted for this purpose which contains, in addition to iron and unavoidable impurities, (in % mass) Si: 2.5-4.0%, C: 0.02-0.10%, Al 0.01-0.065%, N: 0.003-0.015%, optionally up to 0.30% Mn, up to 0.05% Ti, up to 0.3% P, one or more elements from the group S, Se in contents of which the total is at most 0.04%, one or more elements from the group As, Sn, Sb, Te, Bi with contents of up to 0.2% in each case., one or more elements from the group Cu, Ni, Cr, Co, Mo with contents of up to 0.5% in each case, and one or more elements from the group B, V, Nb rich contents of up to 0.012% in each case. The melt composed in this way is then treated in a vacuum system or a ladle furnace in a secondary metallurgical step and subsequently continuously cast into a strand. Thin slabs are separated from the strand obtained in this way and are subsequently heated to a temperature between 1050° C. and 1300° C. in a furnace situate in-line. The dwell time in the furnace is at most 60 mins. After the thin slabs have been heated, the thin slabs are hot rolled into hot strip having a thickness of 0.5-4.0 mm in a multiple-stand hot-rolling train situated in-line. During hot rolling, the first forming pass is carried out at a temperature of 900-1200° C. with a deformation degree of more than 40%. Furthermore, at least the two forming passes following the rolling at 900-1200° C., are rolled in the two-phase mixed region (α-y) during hot rolling. Finally, in the last hot-rolling forming pass, the pass reduction is at most 30%. Following hot rolling, the hot strip obtained in this way is cooled and coiled into a coil. Optionally, the hot strip can subsequently be annealed after coiling or before cold rolling. Afterwards, the hot strip is cold rolled into a cold strip having a final thickness of 0.25 mm to 0.50 mm. The cold strip obtained, is then subjected to recrystallisation and decarburisation annealing. In addition to decarburisation annealing, the strip can also be nitrided in an NH₃-containing atmosphere at temperatures above 850° C. After an annealing separator has subsequently been applied onto the surface of the cold strip subjected to an annealing treatment, the cold strip coated in this way is subjected to a recrystallisation final annealing treatment to form a Goss texture. Equally optionally, the finally annealed cold strip can subsequently also be provided with electrical insulation and finally stress-relieved

In EP 0 378 131 B1, the importance of the average grain size and also its variance is indicated. In addition to an optimum average grain size, it is thus particularly important that the deviation from the average grain size in the sheet is slight. This results from the fact that grain growth processes Lake place in a more uncontrolled manner due to the lower inhibition (Materials Science Forum 204-206 (1996), 623-628). Consequently, under unfavourable processing conditions grains can grow which have no Goss orientation, but at high temperatures are not capable of growth and contribute to fine-grain formation.

Finally, in EP 0 392 534 B1 the eligible atmospheres for the decarburisation annealing are described in detail. In this connection, it is painted out that at the beginning of the decarburisation annealing and nitriding annealing the partial pressure p_(H20)/p_(H2) must be lowered, in order to set a suitable oxide layer. The result of this process is that a satisfactory glass film is formed during coarse grain annealing.

Against this background of the previously explained prior art, the object of the invention was to specify a method, by means of which grain-oriented electrical steel flat products can be produced in a simple mariner wish an optimum uniform distribution of the grain size.

This object was achieved according to the invention by a method which comprises the measures specified in Claim 1.

Advantageous embodiments of the invention are specified in the dependent claims and are explained in detail below together with the general concept of the invention.

In accordance with the previously explained prior art, a method according to the invention for producing a grain-oriented electrical steel flat product intended for electrotechnical applications comprises the following production steps:

-   -   a) producing a steel melt which contains, in addition to iron         and unavoidable impurities, (in % wt.) Si: 2.5-4.0%, C:         0.02-0.1%, Al: 0.01-0.065%, N: 0.003-0.015% and in each case         optionally up to 0.30% Mn, up to 0.05% Ti, up to 0.3% P, one or         more elements from the group S, Se in contents of which the         total is at most 0.04%, one or more elements from the group As,         Sn, Sb, Te, Bi with contents of up to 0.2% in each case, one or         more elements from the group Cu, Ni, Cr, Co, Mo with contents of         up to 0.5% in each case, one or more elements, from the group B,         V, Nb with contents of up to 0.012% an each case,     -   b) casting the melt into a strand in a continuous casting         machine,     -   c) separating at least one thin slab from the cast strand,     -   d) heating the thin slab to a temperature between 1050° C.: and         1300° C.,     -   e) hot rolling the than slab into a hot strap having a thickness         of 0.5-4.0 mm in a hot-rolling train,     -   f) cooling the hot strip,     -   g) coiling the hot strip into a coil,     -   h) cold rolling the hot strip into a cold strip having a final         thickness of 0.15-0.50 mm,     -   i) decarburising and nitriding annealing of the cold strip         obtained,     -   j) applying an annealing separator onto the surface the annealed         cold strip     -   and     -   k) final annealing of the cold strip provided with the annealing         separator to form a Goss texture.

Of course, additional production steps, which are usually required in the conventional production of grain-oriented electrical strips or sheets, can be carried out during production of the electrical steel flat product. These include, for example, a single or multi-stage hot strip annealing treatment carried out between the production steps g) and h), thermal flattening of the cold strip and application of an insulation layer, which can be carried out within the framework of the method according to the invention using and taking into account the parameters known from the prior art.

It is essential for the invention that the cold strip in the course of production step i) “decarburising and nitriding annealing of the cold strip obtained” is subjected to decarburisation and nitriding annealing in at least two stages.

According to the invention, the first stage of this annealing process extends over a first time interval which comprises heating the cold strip starting from a start temperature to a first target annealing temperature and subsequently holding it at this target annealing temperature.

According to the invention, the second stage of the annealing process extends in a corresponding manner over a second time interval, within which the cold strip is firstly heated to a second target annealing temperature and is subsequently held at this target annealing temperature.

According to the invention, the first target annealing temperature is 10-50° C. lower than the second target annealing temperature. At the same time, according to the invention, the duration of the first time interval is 30-70% of the entire duration of the annealing treatment comprising the first time interval ant the second time interval.

The invention proceeds from the finding that a cold strip, in which, on the one hand, the grains have an optimum average grain size, and in which, on the other hand, the deviation of the grain size of the individual trains from the average grain size is slight, can be produced by a “staged annealing process” which is carried out in at least two stages during production step i).

In practice, this can be achieved by conveying the cold strip, obtained after cold rolling, for decarburising and nitriding annealing in a continuous pass through a continuous annealing furnace, which is divided into at least two zones, a target annealing temperature being set according to the invention in the front zone of the furnace first passed through which is 10-50° C. lower than the target annealing temperature in the second zone of the furnace subsequently passed through by the cold strip, wherein the duration of the time interval, within which the first annealing stage takes place, is 30-70% of the entire duration of the decarburising and nitriding annealing. Excessive grain growth of the orientations which are unfavourable for the Goss texture formation is suppressed by means of the temperature difference, predefined according to the invention, between the first and second stages of the decarburising and nitriding annealing and the times provided according to the invention for the two stages of this annealing process. In this way, the cold strip microstructure obtained after the annealing, with the same average grain size which is set by the annealing carried out a higher annealing temperature in the rear furnace zone, has a significantly smaller variance and hence makes homogenous secondary grain growth possible during final annealing carried out at a higher temperature.

In this way, the method according to the invention succeeds in minimising a variance in the grain sizes which arose in the course of the cold-rolling process. Thus, overall, the outcome from the preceding cold process is stabilised with respect to fluctuations in grain size distribution. In this way, after the annealing treatment carried out according to the invention in at least two stages subsequent to the cold rolling, an electrical steel flat product produced according to the invention has a crystallographic texture, by means of which homogenous secondary grain growth is optimally ensured during final high-temperature annealing.

The invention is way combines the procedure known from the low-heating process with modern thin slab manufacture which is carried out according to the known casting-rolling process which is characterised by a continuous manufacturing sequence. As a result, with the procedure according to the invention an electrical steel flat product is obtainable which has optimum magnetic properties in relation to the typical uses for grain-oriented electrical sheets or strips.

When nitriding and decarburising annealing carried out according to the invention in at least two stages is referred to here, this does not mean that combined nitriding and decarburisation necessarily always has to take place in both stages of this annealing process.

Instead, the first stage of this annealing process carried out according to the invention can also be executed as a pure heating stage and the decarburisation and nitriding can take place in the second stage. It is equally possible for decarburisation to be carried out over both annealing stages and for residual decarburisation and nitriding to be subsequently carried out in a further annealing step. Alternatively, the decarburisation and nitriding can take place allocated successively over the at least two stages of the annealing process carried out according to the invention. Finally, it is also possible to let at least one of the annealing stages completed according to the invention to take place without decarburisation or nitriding and only complete the decarburisation and nitriding in an annealing step following the two stages of the annealing process according to the invention.

Accordingly, within the framework of the invention, in production step i)1.i the first and second stages of the annealing process can, in practice, be completed following one another and subsequently a further annealing step carried out, in which the cold strip is subjected to decarburising and nitriding annealing. The first and second stages of the annealing process in production step i) can be carried out taking into account the for these annealing stages according to the invention with respect to the position of the temperature levels and the time slice for the first annealing stage in relation to the overall time for the annealing stages. Afterwards, a further annealing step is then carried out, in which decarburisation and nitriding is carried out in a conventional way. Therefore, overall, in the case of this variant of the invention, at least Three sub-annealing steps are successively completed in the course of production step i), wherein the specifications according to the invention apply for the first two annealing steps and the third step comprising the nitriding can be completed in a conventional way.

Practical tests have shown that optimum properties of an electrical steel flat product produced according to the invention result if the target annealing temperature of the first stage is 10-30° C., lower than the target annealing temperature of the second annealing stage.

There is likewise a favourable effect on the outcome of the annealing step carried out according to the invention in at least two stages if the duration of the first time interval is limited to 30-60% of the entire duration of the annealing treatment.

The cold strip should be heated to the target temperature of the first annealing stage as quickly as possible. During the heating phase of the decarburisation annealing and nitriding annealing, the cold-formed strip initially passes through a recovery. Then, the primary recrystallisation begins. At higher temperatures and with longer annealing times, grain growth processes also occur. In order to provide as much stored energy as possible for the recrystallisation, the temperature range of the recovery should be passed through quickly. For this purpose, one advantageous embodiment of the invention makes provision for the heating rate, at which the cold strip is heated from the start temperature to the first target annealing temperature in the first annealing stage, to be 25-500° C./s. In the case of conventional heating, the heating rate is typically 30-70° C./s. With a view to a particularly good primary recrystallisation and as a consequence thereof optimum production results, it can, however, also be advantageous to set particularly fast heating rates of 200-500° C./s. In practice, such a fast heating rate, particularly in the case of manufacture carried out in a continuous pass, can be achieved by inductive rapid heating taking place at the entrance to the respective continuous furnace, in which the cold strip is heated by the effect of an electromagnetic field induced into the strip.

The invention is explained in more detail below by means of exemplary embodiments.

-   -   Diag. 1 shows a schematic illustration of the temperature course         T over the annealing time t for a conventionally annealed         electrical steel strip (curve A) and an electrical steel strip         according to the invention (curve B);     -   Diag. 2 shows the polarisation at 800 A/m in Tesla for two         differently composed electrical steel sheets S1, S2, plotted via         the ratio t₁/t₂ of the duration of the time interval t₁,         provided for the first annealing stage in the annealing process         according to the invention, to the entire duration t₂ of the         annealing process.

Four steel melts S1-S4 having the compositions specified in Table 1 were continuously cast into a 63 mm thick strand after a secondary metallurgical treatment carried out in a ladle furnace and a vacuum system.

Thin slabs were separated from the strand also in the conventional way. After equalisation annealing in an equalisation furnace at 1165° C., these thin slabs were de-scaled and in the finishing train hot rolled to a final thickness of 2.34 mm and coiled into a coil.

EXAMPLE 1

The hot strips produced in the previously described way were subjected to a two-stage hot-strip annealing process. The annealing temperature in the first stage of the hot-strip annealing process was 1090° C., while the annealing temperature in the second stage was 850° C. Instead of a two-stage hot-strip annealing process, a single-stage hot-strip annealing process with a consistently uniform annealing temperature could have been carried out.

After hot-strip annealing, the annealed hot strip was cold rolled in a single stage with a deformation degree of 87% to a final thickness of 0.285 mm. Sheet samples were separated from the cold strips obtained in this way.

A comparison group A of these sheet samples was annealed in a continuous pass in a continuous annealing furnace. In a first furnace section first passed through, firstly an annealing step lasting 150 seconds was carried out at a temperature of 860° C. under a moist atmosphere consisting of a hydrogen/nitrogen mixture (p_(H20)/p_(H2)=0.50). Then, a second annealing step lasting 30 seconds was carried out in a second furnace section passed through following the first furnace section under a moist atmosphere consisting of an ammonia/hydrogen/nitrogen mixture, in order to bring about residual decarburisation and nitriding. The annealing temperature was constantly 910° C. Corresponding to the embodiment of the invention already mentioned above, which is important in practice, here the annealing process in production step i) of the method according to the invention therefore took place sub-divided into two annealing steps, the first annealing step of which, following the sub division specified according to the invention, was again carried out in two annealing stages, following which conventional decarburising and nitriding annealing was completed as the second annealing step. Overall, production step i) was therefore completed here in three successive parts.

A second group B of sheet samples was, in a corresponding production sequence, firstly annealed in the course of the first annealing step in two successive annealing stages according to the invention and residual decarburisation and nitriding of this second group B of sheet samples was subsequently carried out in a second annealing step. Five variants B.s)-B.e) of the two-stage annealing process according to the invention were tested. In the first annealing stage, taking place over a first duration t₁, in each case a target annealing temperature T₁ was set and in the second annealing stage in each case a target annealing temperature T₂ was set. The entire duration t₂ of the two successively completed annealing stages was in this case 150 s. The first stage of the first annealing step additionally included rapid heating to the respective target annealing temperature T₁ which was carried out at a heating rate of 40° C./sec.

In Diag. 1, the temperature course during annealing in the first annealing step is in each case illustrated via the annealing time t, on the one hand, in a continuous line for the electrical sheet samples of group A produced for comparison and, on the other hand, in a dotted line for one of the variants B.a)-B.e).

Thus, the first two annealing stages of the variant of the method according to the invention explained here by way of example are predominantly used for carrying out decarburisation and are optimised in this respect in terms of gas composition and temperature. The decarburisation annealing takes place in two stages regarding temperature control, namely in such a way that decarburisation is firstly gently carried out in the front section which is first passed through, in order to prevent grain enlargements as far as possible, and in the section subsequently passed through decarburisation is continued and completed at an optimum temperature for the effectiveness of the decarburisation process.

In contrast, the third annealing stage of the method according to the invention is optimised with respect to nitriding. At the same time, residual decarburisation takes place to a minor degree here. The third annealing stage is essentially optimised with respect to nitriding by choosing an optimised gas composition, but it can also mean a temperature adjustment. In Diag. 1, by way of example, a correspondingly carried out temperature control can be recognised by a small temperature jump which occurs after the annealing time t₂ has elapsed.

Specifically, for carrying out the annealing treatment variants B.a)-B.e) according to the invention, the first furnace section of the continuous annealing furnace was divided into two temperature zones of equal length, which the sheet samples to be respectively annealed therefore required 75 s to pass through in each case. Accordingly, these tests, the duration t₁ of the first annealing stage was 50% of the entire duration t of 150 s.

In the first temperature zone of the first furnace section first passed rough by the respective sample, the target annealing temperature was altered from variant to variant when the tests according to the invention were carried out, while in the second temperature zone when the second annealing stage was carried out, in each case a constant target annealing temperature of 860° C. was set. The two annealing stages carried out according to the invention in the first furnace section of the continuous annealing furnace, were in each case carried out under a moist atmosphere consisting of a hydrogen/nitrogen mixture (p_(H20)/p_(H2)=0.50), as with the processing of the sheet samples group A.

Then, as with the treatment of the comparison samples of group A, decarburising and nitriding annealing was carried out over 30 seconds under a moist atmosphere consisting of an ammonia/hydrogen/nitrogen mixture in the second furnace section following the first furnace section. The target annealing temperature was also 910° C. here during this in the second annealing step.

After annealing, the samples were subsequently coated with magnesium oxide and finally annealed under an annealing atmosphere consisting of 50% vol. H2 and 50% vol. N2.

In Table 2, the target annealing temperature T₁ set in the first annealing stage in each case, the difference ΔT between the first target annealing temperature and the target annealing temperature of the second annealing stage, as well as the polarisation J₈₀₀ at 800 A/m, specified in Tesla, and the core loss P_(1.7), specified in W/kg, at a polarisation of 1.7 T and a respective frequency of 50 Hz, are listed for each variant a)-e) of the heat treatment according to the invention. The electrical steel sheets produced according to the invention, regardless of which of the variants a)-e) is used to manufacture them, have proved to have better properties than the samples which undergo an annealing treatment in the conventional way.

EXAMPLE 2

Hot strips produced in the above explained way from melt 1 were subjected to a two-stage hot-strip annealing process at 1130° C./900° C. and hot strips of melt 2 were subjected to a single-stage hot-strip annealing process at 980° C. Afterwards, the hot strips were cold rolled with a deformation degree of 87% in a single stage into 0.285 mm thick cold strips. Sheet samples were separated from the cold strips obtained.

In this case, likewise for comparison, a group A of electrical sheet samples obtained from the cold strips was annealed for a duration of 150 seconds at a temperature of 840° C. in a moist hydrogen/nitrogen mixture atmosphere (p_(H20)/p_(H2)=0.45). Subsequently, annealing was carried out at 860° C. for 30 seconds in a moist ammonia/hydrogen/nitrogen mixture, wherein residual decarburisation and nitriding were carried out. Subsequently, as in Example 1, nitriding and residual decarburisation were carried out at 910° C.

A second group B of samples was annealed in the same atmosphere according to the invention in two stages in the first process part of the continuous furnace used. The temperature of the first furnace cone was set to 810° C. (ΔT=30° C.). Five variants B.a)-B.e) were also shown in this case. The annealing tome t₁, until the target annealing temperature was raised to 840° C. in the second part of the annealing process, was 120 s in the case of variant B.a) (annealing time ratio t₁/t₂=80%), 90 s in the case of variant B.b) (t₁/t₂=60%), 75 s in the case of variant B.c) (t₁/t₂=50%), 45 s in the case of variant B.e) (t₁/t₂=30%) and 30 s in the case of variant B.e) (t₁/t₂=20%). Subsequently, as in Example 1, nitrating and residual decarburisation were also carried out here at 910° C.

The electrical sheet samples were in each case subsequently coated with magnesium oxide and finally annealed under an annealing atmosphere consisting of 50% vol. H2 and 50% vol. N2.

In Diag. 2, the polarisation J₈₀₀ over the annealing time t₁ of the first stage of the annealing process according to the invention is plotted for the samples produced from melts 1 and 2 according to the invention.

EXAMPLE 3

Hot strips of melts 1 and 2 were subjected to a single-stage hot-strip annealing process at 950° C. Subsequently, single-stage cold rolling into cold strip having a final thickness of 0.165 mm was carried out. Sheet samples were separated from the cold strips obtained.

A first group A of the sample sheets separated from the cold strip was annealed for a duration of 130 seconds at a temperature of 880° C. in a moist hydrogen/nitrogen mixture atmosphere (p_(H20)/p_(H2)=0.44). Subsequently, annealing was carried out at 900° C. for 30 seconds in a moist ammonia/hydrogen/nitrogen mixture atmosphere. In the course of this second annealing step, on the one hand, residual decarburisation, and, on the other hand, nitriding were carried out.

A second group B of sheet samples was annealed in two stages under the same atmosphere in the first process part of the continuous furnace used for the tests reported in the present case, wherein during the first annealing stage of the annealing process lasting up to the 70^(th) second) t₁/t₂˜55%) a target annealing temerature of 850° C. was set and then subsequently in the second annealing stage lasting from the 70^(th) second to the 130^(th) second a target annealing temperature of 880° C. was set. Subsequently, as in Example 1, nitriding and residual decarburisation were also carried out here at 900° C. in each case.

After this annealing treatment of the electrical sheet samples, they are each subsequently coated with magnesium oxide and finally annealed under an annealing atmosphere consisting of 50% vol. H2 and 50% vol. N2.

The magnetic properties J₈₀₀ and P_(1.7) of the samples produced according to the invention and for comparison are summarised in Table 3. The superiority of the products produced according to the invention has also been proved here.

EXAMPLE 4

Hot strips produced in the above explained way from melt 3 were subjected to a two-stage hot-strip annealing process at 1070° C./950° C. and cold rolled into cold strip in a single stage having a final thickness of 0.215 mm. Sheet samples were separated from the cold strips obtained.

A first group A of the sheet samples was annealed for duration of 120 seconds at a temperature of 870° C. in an atmosphere consisting of a moist hydrogen/nitrogen mixture (p_(H20)/p_(H2)=0.51). Subsequently, annealing was carried out at 910° C. for 30 seconds under an atmosphere consisting of a moist ammonia/hydrogen/nitrogen mixture, in which, on the one hand, residual decarburisation, and, on the other hand, nitriding took place.

A second group B of sheet samples was annealed according to the invention in a first annealing step divided, into two stages according to the invention in the first furnace section of the continuous annealing furnace used here in a moist hydrogen/nitrogen mixture with p_(H20)/p_(H2)=0.51. In a first annealing stage lasting up to the 65^(th) second the target annealing temperature was set to 850° C., while the target annealing temperature in the second annealing stage, which lasted from the 70^(th) second to the 120^(th) second, was an 870° C. After the end of the first annealing step completed in two stages in the first furnace section on this way, the sheet samples were subjected to nitriding and residual decarburisation at 910° C. in a moist ammonia/hydrogen/nitrogen mixture.

All sheets were subsequently coated with magnesium oxide and finally annealed under an annealing atmosphere consisting of 50% vol. H2 and 50% vol. N2.

In the present example, the first annealing stage of the first annealing step, as in the previously described examples, included rapidly heating the sheet samples to the target annealing temperature of the first annealing stage. In order to show the effect of the heating rates “HR”, in the present Example 4 the heating rates HR were varied in four different test runs with otherwise unchanged conditions.

(Test 4.1: HR=70° C./s; Test 4.2: HR=150° C./s; Test 4.3: HR=300° C./s; Test 4.4: HR=500° C./s;

The magnetic characteristics of the electrical steel shoots obtained in this way are summarized in Table 4.

TABLE 1 Si C Al N Mn S Cu Sn Cr Melt [%] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] [ppm] S1 3.10 470 260 93 1470 78 1950 580 1140 S2 3.32 580 287 105 1390 82 1810 720 780 S3 3.24 720 320 87 1580 92 1470 630 820 S4 2.90 450 348 112 1420 85 1610 1050 930 Details in % wt. or wt. ppm; Remainder iron and unavoidable impurities

TABLE 2 Variant B.a) Variant B.b) according to invention according to invention T₁: 800° C. T₁: 820° C. ΔT: 60° C. ΔT: 40° C. J₈₀₀ [T] P_(1.7) [W/kg] J₈₀₀ [T] P_(1.7)[W/kg] Melt 1 1.865 1.317 1.920 1.038 Melt 2 1.845 1.432 1.909 1.089 Melt 3 1.872 1.238 1.914 1.101 Melt 4 1.853 1.365 1.918 1.030 Variant B.c) Variant B.d) according to invention according to invention T₁: 830° C. T₁: 840° C. ΔT: 30° C. ΔT: 20° C. J₈₀₀ [T] P_(1.7)[W/kg] J₈₀₀ [T] P_(1.7)[W/kg] Melt 1 1.931 1.017 1.922 1.048 Melt 2 1.915 1.056 1.910 1.062 Melt 3 1.924 1.088 1.924 1.087 Melt 4 1.923 1.026 1.919 1.031 Variant B.e) Comparison test not according to the invention according to the invention T₁: 850° C. T₁: 860° C. ΔT: 10° C. ΔT: 0° C. J₈₀₀ [T] P_(1.7)[W/kg] J₈₀₀ [T] P_(1.7)[W/kg] Melt 1 1.910 1.113 1.904 1.123 Melt 2 1.908 1.077 1.899 1.092 Melt 3 1.915 1.092 1.890 1.116 Melt 4 1.899 1.104 1.882 1.107

TABLE 3 Reference 880° C. Invention J₈₀₀ [T] P_(1.7) [W/kg] J₈₀₀ [T] P_(1.7) [W/kg] Melt 1 1.867 0.882 1.899 0.803 Melt 2 1.873 0.853 1.901 0.779

TABLE 4 J₈₀₀ [T] P_(1.7) [W/kg] Test 4.1 HR = 70° C./s 1.879 0.879 according to invention Test 4.2 HR = 150° C./s 1.885 0.853 according to invention Test 4.3 HR = 300° C./s 1.904 0.837 according to invention Test 4.4 HR = 500° C./s 1.903 0.842 according to invention Comparison samples 1.872 0.894 not according to invention 

1. A method for producing a grain-oriented electrical steel flat product, comprising the following production steps: a) producing a steel melt which contains, in addition to iron and unavoidable impurities, (in % wt.): Si: 2.5-4.0%, C: 0.02-0.1%, Al: 0.01-0.065%, N: 0.003-0.015%, and optionally: up to 0.30 % Mn, up to 0.05% Ti, up to 0.3 % P, one or more elements from the group S, Se in contents of which the total is at most 0.04%, one or more elements from the group As, Sn, Sb, Te, Bi with contents of up to 0.2% in each case, one or more elements from the group Cu, Ni, Cr, Co, Mo with contents of up to 0.5% in each case, one or more elements from the group B, V, Nb with contents of up to 0.012% in each case, b) casting the melt into a strand in a continuous casting machine, c) separating at least one thin slab from the cast strand, d) heating the thin slab to a temperature between 1050° C. and 1300° C., e) hot rolling the thin slab into a hot strip having a thickness of 0.5-4.0 mm in a hot-rolling train, 2QB7015.DOC Page 6 f) cooling the hot strip, g) coiling the hot strip into a coil, h) cold rolling the hot strip into a cold strip having a final thickness of 0.15-0.50 mm, i) decarburising and nitriding annealing of the cold strip obtained, j) applying an annealing separator onto the surface of the annealed cold strip, and k) final annealing of the cold strip provided with the annealing separator to form a Goss texture, wherein the cold strip is annealed in at least two stages in the course of production step i), the first stage of this annealing process extends over a first time interval and comprises heating the cold strip starting from a start temperature to a first target annealing temperature and subsequently holding the heated cold strip at the target annealing temperature, the second stage of the annealing process extends over a second time interval, within which the cold strip is firstly heated to a second target annealing temperature and is subsequently held at this target annealing temperature, the first target annealing temperature is 10-50° C. lower than the second target annealing temperature, and the duration of the first time interval is 30-70% of an entire duration of the annealing treatment comprising the first time interval and the second time interval.
 2. The method according to claim 1, wherein the first target annealing temperature is 10-30° C. lower than the second target annealing temperature.
 3. The method according to claim 1, wherein the duration of the first time interval is 30-60% of the entire duration of the annealing treatment.
 4. The method according to claim 1, wherein the heating rate, at which the cold strip is heated from the start temperature to the first target annealing temperature in the first annealing stage, is 25-500° C./s.
 5. The method according to claim 4, wherein the heating rate is at least 200° C.
 6. The method according to claim 5, wherein the cold strip is inductively heated.
 7. The method according to claim 1, wherein in the production step i) the first and second annealing stages are completed following one another and a further annealing step is subsequently carried out, in which the cold strip is subjected to decarburising and nitriding annealing.
 8. The method according to claim 7, wherein the first and second annealing stages are carried out in production step i) as a pure decarburisation annealing process. 